1. Field of the Invention
This invention relates generally to aerial vehicles such as manned or unmanned aerial vehicles, and more particularly to dual-mode aerial vehicles that can fly in both a rotary wing mode and a fixed wing mode.
2. Description of Related Art
Rotary wing aircraft such as helicopters and gyroplanes cannot fly forward at high speeds like fixed wing aircraft. Due to the phenomenon known as the “dissymmetry of lift” caused by the differences in relative air speeds between the advancing blade and the retreating blade, the compressability effect of the advancing blade, the retreating blade stall, and the excessive drag caused by the rotating wings at higher speeds, there is a relatively low speed limit a helicopter or a gyroplane can not exceed. Assuming the aircraft is flying forward, blade retreating occurs when a blade rotates from the aircraft's nose to the tail, which is opposite to the direction of flight. Blade advancing occurs as a blade rotates from the aircraft's tail to the nose, which is the same as the direction of flight. Blade retreating occurs in two quadrants (i.e, half circle, or 180 degrees) in the plane of rotation, and blade advancing occurs in the other two quadrants in the plane of rotation.
A fixed wing aircraft does not suffer the dissymmetry of lift, and is possible to fly at high subsonic, and even transonic or supersonic speeds. However, conventional fixed wing aircraft are not capable of V/STOL and hover. Since the beginning of aviation history, various attempts have been made to create a heliplane, which is defined as an aircraft that combines helicopters' excellent capabilities of V/STOL and hover, with fixed wing aircraft's capabilities of flying at high speeds.
High-speed vertical takeoff and landing aircraft, particularly the tilt-wing, tilt-rotor, trail rotor, folding tilt rotor, stowed-rotor, rotor/wing, and X-wing are well known in the aerospace industry, see NASA technical reports 177578, 177585, 177590, 177592 and 177603. These concepts may be considerably different in design, yet attempt to achieve similar operational goals of vertical takeoff and landing, and high speed forward flight. Current vertical and short takeoff and landing (V/STOL) aircraft designs attempt to have both qualities: efficient hovering and high-speed forward flight. Demonstrated designs appear to be an unsatisfactory compromise between the two.
One known V/STOL aircraft, the tilt-wing, uses large oversized conventional propellers driven by engines. These engines are attached to a wing that can be tilted from the horizontal position for forward flight, and to the vertical position for vertical takeoff and landing. The tilt-rotor, such as the Bell/Boeing V-22 Osprey, behaves similarly except that only the rotors and engines tilt, not the wing. These aircraft compromise both hovering efficiency, because of high-disk-loading, and forward speed. When the rotors are tilted forward for forward flight they become inefficient impellers above about 300 miles/hour. The trail rotor and folding tilt rotor have the potential to overcome the inefficiency of the tilt rotor by having low-disc-loading during hover, and high-fixed-wing-loading during high speed flight, however, since the rotors have to be tilted forward or aft 90 degrees before the rotors are stopped in a vertical plane of rotation, these two concepts have an even more complex and lengthy conversion process than the tilt rotor.
Another concept known as the stowed-rotor behaves like a conventional helicopter for vertical takeoff and landing. For high speed forward flight the rotors are slowly stopped and stowed out of the way of the airstream to reduce drag, while a set of conventional fixed wing airfoils assume primary lift. U.S. Pat. Nos. 3,515,500 to Nachod (1968) and 4,059,247 to Prewitt (1976) show the complexity involved in stopping and folding the rotors. These aircraft have hover efficiencies approaching a helicopter, yet require the additional weight of a fixed wing since the stopped rotor does not provide any lift.
Since the current invention is a horizontally stopped/lifting heliplane, prior arts involving horizontally stopped/lifting rotor will be given special attention here. Here “horizontally stopped” means the rotor is stopped in a substantially horizontal plane of rotation, and “lifting” means the stopped rotor provides some lift from at least one blade of the rotor during at least part of the fixed wing flight. This broad definition of “lifting” is intended to not only include aircraft that gets lift from the stopped rotor, but also to include aircraft that includes a stage where the stopped rotor provide some lift, but in some other stages of the fixed wing flights the blades may not provide any lift. A major challenge in horizontally stopped/lifting rotor aircraft is related to, besides the stopping of the rotor itself, how to deal with the reverse flow problems associated with any blade in the retreating region. If a retreating blade has a typical airfoil shape with a tapered trailing edge and a rounded leading edge, after it is stopped, reverse flow from the tapered edge to the rounded edge can cause flow separation and aerodynamic problems. Known prior arts attempt to resolve the reverse flow problems of any retreating blade after stopping the rotor or rotors in one way or another but suffer various disadvantages.
U.S. Pat. No. 3,207,457 issued to Kisovec discloses an aircraft with two counter-weighted single-bladed rotors mounted at the tips of the main fixed wings. The blades are used as wing extension while the counter-weights are stowed during fixed wing flight. This invention has low figure of merit. The single-bladed rotor creates balance-of-force and oscillation problems in rotary flights due to the single-bladed rotors.
U.S. Pat. No. 3,494,707 issued to Kisovec discloses a “Rotafix” aircraft with two 2-bladed rotors mounted at the tips of the main fixed wings. The outboard blades are used as wing extension while the inboard blades are stowed at the trailing position during fixed wing flight. In order to achieve low-disc-loading during rotary flight, the rotor blades tend to be relatively long, which gives the extended wing a very high aspect ratio, excessive wing area, and a poor L/D in high speed cruise, see NASA technical report 177585. Due to the mechanical limitation in the rotor construction, this invention lacks the capabilities to place both blades of each rotor at the trailing position, which is desired in order to increase wing loading during high-speed flights. It also requires a complex mechanism to rotate the blade at the trailing position by 90 degrees about its pitch axis while keeping the outboard blade substantially horizontal.
One concept known as the stopped rotor X-wing aircraft behaves like a conventional helicopter for vertical takeoff and landing, having low-disk-loading. To achieve high-speed forward flight the four main rotor airfoils are slowly stopped and fixed in an “X” position in the horizontal plane, forming 45 degree swept wing angles (two airfoils are forward swept 45 degrees, and the other two airfoils are aft swept 45 degrees). The stopped rotor airfoils provide primary lift for forward flight, eliminating the need for additional fixed wings. Since two of the four main rotor airfoils are essentially flying backwards in the fixed wing position (relative to their rotary wing airfoil position), a complicated air circulation control system is required for each airfoil to achieve lift in both the rotary and fixed wing operation. This causes the airfoil leading edge to be identical to the trailing edge. U.S. Pat. No. 4,711,415 to J. A. Binden uses high-pressure air blown over the airfoil leading and trailing edges, via span-wise running slots, to achieve circulation control. The rotor systems research aircraft X-wing (RSRA/X-wing), based on principles of this patent, and built by Sikorsky Aircraft Corporation, was never able to demonstrate rotary wing flight or transition from rotary wing to fixed wing flight, or vise versa. Complexity and number of the mechanisms associated with main rotor airfoil circulation control, and questionable reliability of successfully transitioning between rotary wing and fixed wing flight, and vise versa, caused the program to be abandoned.
U.S. Pat. No. 5,405,104 “Stopped Rotor Aircraft Utilizing a Flipped Airfoil X-Wing” issued to J. B. Pande discloses an aircraft having a 4-blade rotor. During the rotary wing flight the blades are operated like a conventional helicopter. The four blades are stopped to serve as fixed wings for high-speed flight. During transition to fixed wing flight two adjacent blades are flipped 180 degrees about their pitch axis such that all blades have leading edges in the correct orientation for a particular flight mode. This invention has the disadvantage of structural and control difficulties associated with the flipping of blades. During the flipping of blades, the blades have to go through a vertical position where the cross section area exposed to the airstream is maximized, causing a tremendous aerodynamic drag. Although the spike of drag only lasts for a short period of time, it is enough to cause structural and yaw control stability issues.
U.S. Pat. No. 5,454,530 “Canard Rotor/Wing” issued to J. W. Rutherford et al is an inventive aircraft utilizing a jet-propelled rotor/wing, preferably having two blades, to achieve V/STOL and hover when the aircraft is in the helicopter mode. During the transition from the helicopter mode to fixed wing airplane mode, the aircraft utilizes its canards and horizontal tail to generate substantially all of the lift so that the rotating rotor/wing can be unloaded. Afterwards, the rotating speed of the rotor/wing is slowed down and eventually stopped and locked to function as the left and right wings. When flying in fixed wing airplane mode, the rotor/wing can operate like an oblique wing to maximize flight efficiency at different speeds. This invention has a disadvantage of high drag and low lift efficiency. The leading edge of the retreating blade becomes the trailing edge after stopping. In order for the retreating blade to avoid flow separation problems due to reverse flow after stopping, the blade airfoil cannot adopt the most efficient shape of a typical airfoil, and instead has to be compromised to adopt the less efficient shape with a rounded trailing edge identical to the leading edge, which leads to high drag and low lift efficiency in both rotary wing mode and fixed wing mode. Typical airfoils for subsonic flights have a characteristic shape with a rounded leading edge, followed by a tapered (sharp) trailing edge to reduce drag. In addition, the collective pitch control is further complicated due to the fact that the retreating blade's leading and trailing edges switch positions after stopping.
U.S. Pat. No. 2,879,013 “Convertible Aircraft” issued to G. P. Herrick is an invention utilizing a straight rotor/wing and a straight fixed wing. In the helicopter mode, the straight rotor/wing rotates like a two-blade rotor. In the airplane mode, the rotor/wing is stopped and together with the fixed wing, forming a configuration similar to a biplane. The two straight wings limit the efficiency in high-speed flights due to drag divergence. For the same reason as in the CRW, the rotor/wing has to adopt an airfoil cross section with a trailing edge substantially identical to the leading edge, which increases drag in both flight modes.
U.S. Pat. No. 3,327,969 “Convertible Aircraft” issued to R. E. Head invents an aircraft utilizing a jet-propelled rotor/wing. In the helicopter mode, the rotor/wing rotates and generates lift with three stub blades installed on a large center body of the rotor/wing. The large center body was sized to provide the necessary lift during conversion to fixed-wing flight as the rotor slowed and stopped rotating. At airplane mode, the center body and two of the three stub blades generate lift for the aircraft. This invention has a disadvantage of low flying efficiency and low lift efficiency. The centerbody has poor L/D ratio. In helicopter mode, the relatively small and low aspect ratio stub blades are not efficient in generating lift, therefore requiring high induced power to generate the necessary lift. In the fixed wing airplane mode, the large centerbody creates large drag force. For the same reason as in the CRW, the two lifting stub blades has to adopt an airfoil cross section with a trailing edge substantially identical to the leading edge, which increases drag in both flight modes.
U.S. Pat. No. 7,014,142 “Low-Drag Rotor/Wing Flap” issued to Barocela et al. discloses an inventive blade having a body portion and a flap portion. The flap is movably coupled to the body portion. The movable flap enables the blade edge where the flap is attached to transform between a rounded leading edge and a tapered trailing edge. This invention acknowledges the high drag and low lift efficiency associated with the rounded trailing edge in “Canard Rotor/Wing”, and attempts to resolve the reverse flow problems of the retreating blades by using at least one movable flap. This invention has a disadvantage of complexity, as well as the disadvantages of structural and control difficulties associated with flipping the flaps. In order to gain the benefits of reduced drag during both rotary and fixed wing modes, both edges of at least one blade have to be equipped with flaps. The movable flaps require extra mechanical devices to provide power and control during the conversion between the two flight modes. During the conversion only one of the two blades has to transform the leading edge into the trailing edge and vice versa. The flaps on that blade have to be swung 180 degrees from one horizontal position to vertical position and then to the opposite horizontal position. Flipping the flaps creates similar structural and yaw control difficulties as flipping the blades in U.S. Pat. No. 5,405,104. When the flaps are at their vertical position, the drag force created by oncoming airstream is significant if the blade under conversion is at the transverse position relative to the fuselage. One possible way to overcome the problems is to perform the flipping when the blade is pointing in the longitudinal direction of the fuselage rather than in the transverse direction. After the transformation of the one blade, the stopped rotor can be rotated to the generally transverse position to act as a fixed wing. However, this process increases the complexity of the motor locator control, and lengthens the duration of the conversion between the rotary wing mode and fixed wing mode. The duration of conversion is critical when the fixed wing needs to be quickly transformed back to the rotary wing, for example, in case of engine failure.
What is needed, therefore, is an aerial vehicle which successfully combines V/STOL, hover and low speed capabilities of a helicopter, and the high-speed flight capability of a fixed wing aircraft, is capable of smoothly, efficiently, and safely converting from one flight mode to another, and is capable of using the most efficient airfoils for rotor/wing to reduce drag and improve flying efficiency.
All helipanes utilizing horizontally stopped/lifting rotor must face the reverse flow problems associated with any blade in the retreating region. Known prior arts attempt to resolve the problems using one or more of the following methods, but suffer various disadvantages: 1) use two counter-weighted single-bladed rotors, stow the counter weights and use the single blades as wing extensions; 2) use dual 2-bladed rotors, stow away the inboard and retreating blades, and use the outboard blades as wing extensions; 3) make the blade airfoil reversible, which means the airfoil's leading and trailing edges are identical; 4) use circulation control; 5) flip any retreating blade; 6) flip flaps on any retreating blade.
All of the above-mentioned prior art references are hereby incorporated by reference in full.
The current invention resolves the reverse flow problems of any retreating blade by using two side-by-side counter-rotating rotors, a unique rotor design allowing the angular spacing of the blades of each rotor to be variable in flight, and an innovative “segregated X-wing” blade configuration to provide lift. The side-by-side dual rotor arrangement is sometimes called “transverse array”, or “transverse” dual rotors. The side-by-side dual rotors arrangement includes two configurations: non-intermeshing and intermeshing.
The above mentioned and other objects, features, and advantages of the invention and the manner of obtaining them will become apparent, and the invention itself will be best understood, by reference to the following description taken in conjunction with the accompanying illustrative drawings.